Hi Martin,
Microsoft does not support Inserting/Updating data directly using SQL.
however I have worked with some customer which has done this mainly because of data load so if you loading 1-2Million records in short span then SQL gives better performance, in this scenario if you doing in SQL you could create your own GUID and then pass
same GUID for same record in AccountBase and AccountExtensionbase table.
Again it is preferred to use support approach so my suggestion is to explore others supported options for this (e.g. SSIS packages using kingsWaysoft, custom code using SDK API etc.)
MayankP
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